Most of the conventional electrophotographic photoreceptors use a light-sensitive layer containing an inorganic photoconductor such as selenium, zinc oxide, cadmium sulfide or the like as the main component. But none of these photoreceptors have satisfactory heat resistance or durability. Further, great difficulties are met in the manufacture and handling of these photoreceptors due to their toxicity.
Electrophotographic photoreceptors using a light-sensitive layer containing an organic photoconductive compound are also known, and they are getting an increasing amount of researchers' attention these days because they are fairly easy to manufacture, low in production cost, easy to handle, and superior in heat stability to the selenium photoreceptor and the like.
As such organic photoconductive compound, poly-N-vinylcarbazole is well known, and an electrophotographic photoreceptor having a light-sensitive layer that contains as the main component a charge transfer complex formed from the poly-N-vinylcarbazole and a Lewis acid such as 2,4,7-trinitro-9-fluorenone is currently used in industry. A two-layer or dispersed type photoreceptor wherein the carrier generating function and the carrier transporting function are fulfilled by two different materials is known. For example, a product having a light-sensitive layer wherein a carrier generation layer made of a thin amorphous selenium film and a carrier transporting layer made of poly-N-vinyl carbazole are combined is commercially used. But the poly-N-vinyl carbazole is not highly flexible and a coat made of it is rigid and brittle and is highly sensitive to cracking or exfoliation. Therefore, a photoreceptor using this compound does not have great durability. If a plasticizer is added to solve this problem, the residual potential is increased as the photoreceptor is subjected to electrophotographic process, and the accumulated residual potential due to cyclic use causes increased fog in the copied image.
Most organic photoconductive compounds of low molecular weight have no film-forming properties and are used in combination with suitable binders. So they are preferred in that the physical properties of the film and the electrophotographic characteristics of the copy can be controlled to some extent by changing the type of the binder used or its proportion. However, only limited types of organic photoconductive compounds have high miscibility with binders and, in fact, there are not many compounds that can be incorporated in the light-sensitive layer of a photoreceptor.
For instance, U.S. Pat. No. 3,189,447 describes 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole, and this compound has low miscibility with binders conventionally used in the light-sensitive layer of a photoreceptor. When that compound is mixed with a binder such as polyester or polycarbonate in the ratio necessary for producing the desired electrophotographic characteristics, the oxadiazole crystallizes in the light-sensitive layer at 50.degree. C. or higher, with the result that its electrophotorgraphic characteristics such as charge retention and sensitivity are impaired.
U.S. Pat. No. 3,820,989 discloses a diaryl alkane derivative and this compound has no problem with its miscibility with binders. However, the compound has low stability to light, so when it is incorporated in the light-sensitive layer of a photoreceptor for cyclic transfer xerography wherein it is subjected to repeated charging and exposure, the sensitivity of that layer is gradually decreased and the residual potential increased. This means the photoreceptor is inferior in durability. Therefore, the state of the art has no organic photoconductive compound available that has preferred characteristics for use in commercial production of electrophotographic photoreceptors.